Nucleus by suchenfz

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									        Some Cellular Functions


• Protein synthesis
• DNA replication
• Cell division
                DNA History
Discovery of the double helix
  – by 1900: components of DNA were known
  – by 1953: x-ray diffraction determined geometry of
    DNA molecule
  – Nobel Prize (1962): __________________________
    but not to Rosalind Franklin who died of cancer at 37
     Organization of the Chromatin
• Humans - 46 molecules
  of DNA + their proteins
  form chromatin
   – looks like granular thread
• DNA molecules
  compacted
   – coils of nucleosomes
     (histone clusters)
   – twisted into a coil that
     supercoils
             Nucleotide Structure
• Nucleic acids like DNA are
  polymers of nucleotides
• Nucleotides consist of:
DNA Structure: Twisted Ladder
              Nitrogenous Bases
• Purines - double carbon-
  nitrogen ring
  – guanine
  – adenine
• Pyrimidines - single
  carbon-nitrogen ring
  – uracil - _____________
  – thymine - ______________
  – cytosine - both
    Complementary Base Pairing
• Nitrogenous bases form
  hydrogen bonds




                             Sugar-phosphate backbone




                                                                         Sugar-phosphate backbone
• Base pairs
  – A-T and C-G
• Law of complementary
  base pairing
  – one strand determines
    base sequence of other


                                                        Segment of DNA
                DNA Function
• Code for protein (polypeptide) synthesis
• What is a Gene? - sequence of DNA nucleotides
  that codes for one polypeptide
• Genome - all the genes of one person
  – humans have estimated 30,000 genes (~3 billon
    nucleotides)
  – much of DNA is noncoding – either “junk” or
    organizational
  – human genome project completed in 2003
    RNA: Structure and Function
• RNA much smaller than DNA
  – Three types of RNA: _________________________
• Only one nucleotide chain
  – ribose replaces deoxyribose
  – uracil replaces thymine
• Essential function
  – interpret DNA code
  – direct protein synthesis in the cytoplasm
 Genetic Control of Cell Action through
           Protein Synthesis
• DNA directs the
  synthesis of all cell
  proteins
• Different cells
  synthesize different
  proteins
     Preview of Protein Synthesis
• Transcription
  – mRNA is formed next to an activated gene
  – mRNA migrates to ___________________
• Translation
  – mRNA code is “read” by rRNA as amino acids
    are assembled into a protein molecule
  – _______________ delivers the amino acids to the
    ribosome for assembly into a protein
                    Genetic Code
• Only 4 nucleotides (A,T,G,C) code for 20 amino acids
• Base triplet:
   – found on DNA molecule (ex. TAC)
   – sequence of 3 nucleotides that codes for 1 amino acid
• Codon:
   – “mirror-image” sequence of nucleotides in mRNA (ex AUG)
   – ______ possible codons (43)
      • often 2-3 codons represent the same amino acid
      • start codon = __ __ __
      • 3 stop codons = UAG, UGA, UAA
                  Transcription
• Copying genetic instructions from DNA to RNA
  – RNA polymerase binds to DNA & opens helix
  – bases transcribed from 1 DNA strand into pre-mRNA
      • if C on DNA, G is added to mRNA
      • if A on DNA, U is added to mRNA, etc.
  – rewinding of DNA helix
• Pre-mRNA is unfinished
  – “nonsense” portions _________ removed by enzymes
  – “sense” portions _______ reconnected & exit nucleus
Transfer RNA (tRNA)
Translation of mRNA
Review: DNA & Peptide Formation
Protein Packaging & Secretion




     Posttranslational Modification
                 DNA Replication
• Law of complimentary base pairing allows building
  of one DNA strand based on the bases in 2nd strand
• Steps of replication process
  – DNA helicase opens short segment of helix
  – DNA polymerase
     • strands replicated in opposite directions
              DNA Replication
• __________________________ replication
  – each new DNA molecule has one new helix with the other
    helix conserved from parent DNA




46 chromosomes replicated in 6-8 hours by 1000’s of
polymerase molecules
DNA Replication: Errors and Mutations
• Error rates of DNA polymerase
   – in bacteria, 3 errors per 100,000 bases copied
   – every generation of cells would have 1,000 faulty proteins
• Proofreading and error correction
   – a small polymerase proofreads each new DNA strand and
     makes corrections
   – results in only 1 error per 1,000,000,000 bases copied
• Mutations - changes in DNA structure due to replication
  errors or environmental factors
   – some cause no effect, some kill cell, turn it cancerous or cause
     genetic defects in future generations
Cell Cycle
        Timing of Cell Division
Cells divide when:
• Have enough cytoplasm for 2 daughter cells
• DNA replicated
• Adequate supply of _____________
• Growth factor stimulation
• _______________ due to neighboring cell death
Cells stop dividing when:
• Loss of growth factors or nutrients
• _______________________
                       Mitosis
• Process by which one cell divides into 2 daughter
  cells with identical copies of DNA
• Functions of mitosis
  –   embryonic development
  –   tissue growth
  –   replacement of old and dead cells
  –   repair of injured tissues
• Phases of mitosis (nuclear division)
  – prophase, metaphase, anaphase, telophase
               Cytokinesis
• Division of cytoplasm / overlaps telophase
• Myosin pulls on microfilaments of actin in the
  membrane skeleton
• Causes crease around cell equator called
  cleavage furrow
• Cell pinches in two
• Interphase has begun
                     Cancer
• Tumors (_______________)
  – abnormal growth, when cells multiply faster than
    they die
  – oncology is the study of tumors
• Benign
  – ________________________________, stays local
  – potentially lethal by compression of vital tissues
• Malignant
  – unencapsulated, metastatic (90% of cancer deaths)
                 Causes of Cancer
• Carcinogens - estimates of 70 -
  80% of cancers from
  environmental agents
  – chemical
    • cigarette tar, food preservatives
  – radiation
    • UV, -  particles,  rays
  – viruses
    • type 2 herpes simplex - uterus,
    • hepatitis C - chronic inflammation
                      Mutagens
• Trigger DNA (gene) mutations
  – cell may die, be destroyed by immune system, or produce a
    tumor
Defenses against mutagens
• Scavenger cells
  – remove them before they cause genetic damage
• Peroxisomes
  – neutralize oxidizing agents & free radicals
• Nuclear enzymes
  – repair DNA
• Tumor necrosis factor (TNF)
  – from macrophages and certain WBCs destroys tumors
 Malignant Tumor (Cancer) Genes
• ___________________
  – mutated form of normal growth factor genes
     • may cause excessive production of growth factors
     • stimulate neovascularization of tumor
  – abnormal growth factor receptors
     • sends constant divide signal to cell
• Tumor suppressor genes
  – inhibit development of cancer
• Damage to one or both removes control of cell
  division

								
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